The present invention relates to a heat-resistant and corrosion-resistant weld metal alloy having superior high temperature ductility and high temperature strength.
More particularly, the invention is concerned with an austenitic welded structure for high temperature use and, still more particularly, to an austenitic welded structure having superior high temperature ductility, high temperature strength, corrosion resistance and fatigue life, and effective in preventing thermal stress cracking, suitable for use in hot spots in various plants such as chemical plants, thermal power plants, nuclear power plants, boilers and so forth.
Hitherto, austenitic stainless steels and heat-resistant alloy steels have been used as the material of structures used at a high temperature at which creep is liable to occur. Particularly, as the base metal of the structure at a high temperature higher than 600.degree. C., an NCF steel (0.08%C-20%Cr-32%Ni-0.3%Ti-0.3%Al) is often used because of its superior creep rupture strength. (Throughout the specification, the compositions of materials are expressed in terms of percent by weight (wt%) unless otherwise specified.) Generally, chemical plants, thermal power plants, nuclear power plants and boilers have lot of welded structures. The fabrication of welded structure using the NCF steel as the base metal is made by means of a welding rod such as of Incoloy 138 (28%Cr-38%Ni-4%Mo-1%W-balance Fe), Inconel 182 (15%Cr-0.5%Ti-2%Nb-7%Mn-8%Fe-balance Ni), Inconel 112 (20%Cr-8%Mn-3.5%Nb+ta-7%Fe-balance Ni) and Inco Weld A (15%Cr-2%Nb-1.5%Mo-8%Fe-balance Ni). The weld metal formed with these welding rods, however, have only a small ductility at high temperature and exhibits a heavy embrittlement due to a change in the structure during the use at the high temperature. In addition, cracks are liable to be formed along the boundary in the weld zone due to a large difference in the thermal expansion coefficients attributable to the difference in compositions between the weld metal and the structural base metal of NCF steel. In order to overcome this problem, it has been attempted to fabricate the welded structure by using such a welding rod as would produce a weld metal of which thermal expansion coefficient becomes identical with that of the base metal of NCF steel, i.e. of which composition becomes identical with that of the base metal of NCF steel. In the welded structure fabricated with such a welding rod, however, the fluidity of the molten metal is deteriorated by formation of oxides of Ti and Al contained in the base metal during the welding to undesirably increase the occurrence of weld defects. It is of course possible to eliminate the Ti and Al contents in the base metal but the elimination of Ti and Al undesirably lowers the strength at high temperature.